This ESO workshop, which took place in September 1995 on a topic that at a first glance could be considered rather specialized, attracted an unpre­ dictably large number of scientists. This certainly reflects the importance of this field, which has lost its seemingly esoteric character, in a wider astro­ physical context. To give as much room as possible in these proceedings to the targeted talks, no presentation of the Very Large Telescope Observatory has been included. All readers missing such a presentation are reminded that up- to­ date in-depth information about the VLT status is available electronically.1 Papers were given concerning observations in the entire electromagnetic